The photoconduction process of an electrophotographic photoreceptor is comprised of (1) the step of generating charges by exposure to light, and (2) the step of transporting charges.
Steps (1) and (2) may be effected using the same substance such as a selenium photosensitive plate. However, steps (1) and (2) may be carried out using different substances, for example, the combination of amorphous selenium and poly-N-vinylcarbazole is well-known. Carrying out steps (1) and (2) separately using different substances has advantages that materials to be employed in a photoreceptor can be selected from a wide variety of materials. Accordingly, electrophotographic characteristics, such as photosensitivity, acceptable potential and so on, are improved, and it becomes also possible to select materials which are well-suited for producing photoreceptors using a coating technique and such materials can be selected from a wide range of substances.
Substances which have so far been employed as photoconductive materials of photoreceptors to be used in electrophotographic systems include such inorganic substances as selenium, cadmium sulfide, zinc oxide and so on.
Electrophotography, as Carlson has already disclosed in U.S. Pat. No. 2,297,691, utilizes photoconductive materials which can change their electric resistance with exposure amounts they receive during imagewise exposure, have an electrically insulating property in the dark, and can be provided on supports using coating techniques. In general a photoconductive material receives dark adaptation processing for a proper period of time and is then forced to create uniform charge on its surface in the dark. The material is then subjected to imagewise exposure through such an irradiation pattern that it has the effect of reducing the surface charge depending upon relative energies contained in its constituent parts, respectively. The remaining surface charge or electrostatic latent image on the surface of the photoconductive layer (photo-receptive layer) is subsequently converted to a visible image by allowing it to come into contact with a proper charge-detecting and -developing substance, that is, toner.
Toner, whether it is contained in an insulating liquid or in a dry carrier, can adhere to the surface of the photo-receptive layer corresponding to a charge pattern. The toner which sticks to the charge pattern can be fixed thereon by known means, for example, by application of heat, pressure or solvent vapor. Alternatively, an electrostatic latent image can be transferred onto the second support (e.g., paper, films or so on). Similarly, an electrostatic latent image transferred on the second support can be developed thereon. The electrophotographic method is one of the known image-forming processes which is designed to form an image in the above-described manner.
Fundamental characteristics required of photoreceptors in electrophotography as described above are (1) a proper height of surface potential which can be created thereon by charging in the dark, (2) a high charge retentivity in the dark which can be aquired by reduced dark decay of surface charge, (3) rapid decay of surface charge by light-exposure, and so on. In fact, the above-described, conventionally used inorganic substances have many advantages and at the same time, suffer from various defects. For instance, selenium, which is prevailingly used as a photoreceptor at present, fully satisfies the above-described requisites (1) to (3). However, it suffers from the defects that conditions for manufacturing selenium photoreceptors are difficult to create and maintain. Therefore, the cost of manufacturing them becomes high, selenium photoreceptors must be handled with care due to their high sensitivities to heat and mechanical impact, and difficulties arise in shaping them into a belt-form due to lack of flexibility. Cadmium sulfide and zinc oxide are each used as a photoreceptor in the form of a dispersion in a resin which functions as a binder. Such a photoreceptor is disadvantageous mechanically, that is, its surface smoothness, harness, tensile strength, frictional resistance and the like are too low to use them repeatedly.
Recently, electrophotographic photoreceptors utilizing organic substances have been proposed with the intention of removing defects inherent in the above-described inorganic substances, and some of them have already been submitted to practical use. Specific examples include a photoreceptor constituted with poly-N-vinylcarbazole and 2,4,7-trinitrofluorene-9-one (as described in U.S. Pat. No. 3,484,237), poly-N-vinylcarbazole sensitized with dyes of pyrylium salt type (as described in published examined Japanese Patent Application No. 25658/73 corresponding to U.S. Pat. No. 3,617,268), photoreceptors containing organic pigments as their principal components (as described in published examined Japanese Patent Application No. 1194/81 published unexamined Japanese Patent Application No. 133445/78, European Pat. No. 34,498 and U.S. Pat. No. 3,898,084), photoreceptors containing eutectic complexes consisting of dyes and resins as their principal components (as described in published unexamined Japanese Patent Application No. 10785/72 corresponding to U.S. Pat. Nos. 3,732,180 and 3,684,502) and so on.
Those organic electrophotographic photoreceptors are improved in mechanical characteristics and flexibility to some extent compared with those of the above-described inorganic ones. However, taking into account the present state of organic photoreceptors, for example, the organic ones are generally inferior to the inorganic ones in terms of photosensitivity. Further, they are unsuitable for repeated use. Accordingly, those which fully satisfy all of requirements for a photoreceptor to be employed in electrophotography have not yet been obtained.